White light emitting diodes (LEDs) developed as devices for illumination are realized by combination of the three primary colors of light, and generally employ combination of a blue LED chip and yellow phosphors. The yellow phosphors representatively include Ce-containing yttrium aluminum garnet (YAG)-based phosphors, and various phosphors such as terbium aluminum garnet (TAG)-based, silicate-based, and nitride-based phosphors are being developed and used as the yellow phosphors. In addition, to improve color reproducibility of the white light emitting diodes, a method for combining red and green phosphors is also being developed.
However, since the YAG-based phosphors used from the earliest stage among phosphor materials for white LEDs have various merits such as material stability, high photo-conversion efficiency and the like, the YAG-based phosphors are used for most of white LED products which are currently commercially available. Such phosphors are used in a powder form and are mixed with an epoxy or silicone binder and then coated onto an upper side of a blue LED. The white LEDs are used for LCD backlights, auxiliary illumination, various pilot lamps and the like, and white LEDs for main illumination are also developed and commercially available.
Although the YAG phosphors used for white LEDs for illumination have a merit of long lifespan due to high fluorescence efficiency and stable bonding structure thereof, a spectrum of yellow light converted and emitted by the YAG phosphors absorbing blue light exhibits color imbalance in which a red light band is weaker than a green light band. Thus, when the white LEDs are realized by combination of the YAG phosphors and a blue LED chip, bluish white light, that is, cold white light having a high color temperature is realized. Due to overall color ratio imbalance, when the white LEDs are used as an illumination device, the white LEDs exhibit deterioration in color reproducibility, thereby deteriorating reliability of the illumination device. Although this phenomenon occurs due to inherent properties of the phosphors regardless of the kinds of phosphor supporting materials, it is easy to adjust optical properties of the white LEDs by modifying a composition of a glass material when the glass material is used as a phosphor binder.